V. Alveolar process and alveolar bone
B. BONE REMODELING
Despite its solid appearance, bone is in a constant state of remodeling. This means that at all time some parts of the jaw bone are being resorbed, while other parts are growing by apposition of new bone. This process requires some coordination between resorption and apposition so that the normal function of the bone can be maintained. Remodeling of the alveolar process allows the normal migration of teeth in a mesial direction, or mesial drift, as their interproximal surfaces wear down. It also allows for orthodontic tooth movement and wound healing.
Fig. 124: Mesiodistal section through maxillary canine (C) and lateral incisor (I). The right side of the photomicrograph is the mesial direction. Therefore, one may expect to see histological evidence of mesial drift, with teeth tending to move slowly through bone from the left to the right side of the photomicrograph.
Fig. 125: Enlarged view of the interdental septum (IDS) between the canine and the lateral incisor. The mesial and distal directions are labeled D and M, respectively. Most of the tooth support in the center of the jaw is provided by cancellous bone.
Fig. 126: Magnified view of interdental septum shown in Fig. 125. Note the large bone marrow spaces (BMS) in the cancellous bone of the interdental septum. The distal side (D) of the septum has the characteristics of as a predominantly bone-resorbing surface (BRS), whereas the mesial (M) side has the typical features of a bone-forming surface (BFS).
Fig. 127: Mesial side of interdental septum shown in Fig. 126. The bone surface facing the periodontal ligament (PDL) has characteristic features of a predominantly bone-forming surface. These include the presence of bundle-bone (BB), some of which is being remodeled into osteons or Haversian systems (HS). At this magnification, bundle bone appears to be composed of bone lamellae separated by distinct cement (appositional) lines. Other features, not seen at this magnification, include osteoblasts lining the bone surface and Sharpeys' fibers orientated perpendicularly to the bone surface.
Fig. 128: Distal side of interdental septum shown in Fig.126. The bone surface facing the periodontal ligament (PDL) has characteristic features of a predominantly bone-resorbing surface. These may include the absence of a well-defined layer of bundle bone, the presence of a scalloped bone surface, the presence of resorbing lacunae (RL) that may or may not contain bone-resorbing cells, and the presence of a reversal line (RL) that indicates the location of the most advanced resorbing front prior to the subsequent formation of a thin seam of bone to reanchor the periodontal ligament fibers.
The actual histologic features of a predominantly bone-resorbing surface will vary, depending on the physiologic status of that surface. Since bone resorption occurs in a cyclical manner, the histologic features will change depending on the cycle. Thus a predominantly bone-resorbing surface may show evidence of active bone resorption, it may be in a resting phase, or it may actually show evidence of ongoing bone deposition, although there are usually signs that this is only a short, transient phase.
Fig. 129: Bundle bone (BB) on the mesial ( a bone-forming) surface of an interdental septum. At this magnification, pale staining Sharpey's fibers (SF) can be detected. Cyclical deposits of bone, formed by surface apposition, are separated by darker staining cement lines (CL).
Fig. 130: Actively resorbing bone surface on the distal aspect of an interdental septum. Several resorption lacunae (RL) - also known as Howship's lacunae - are visible, some with multinucleated osteoclasts. There is no evidence of any reversal line, as the resorptive process destroyed any trace of the former resorption front and no evidence exists, as yet, of any recent new bone formation.
Fig. 131: Bone surface during a resting phase (RBS). The scalloped outline of the bone surface and the absence of bundle bone over the partially resorbed osteons (HS) indicate that this is a predominantly resorbing bone surface. This phase is generally followed by a phase of bone deposition of short duration that helps reestablish the anchorage of the bone to the periodontal ligament
Fig. 132: Predominantly bone-resorbing surface during a phase of bone apposition. The reversal line (Rev) is seen as a distinct, dark-staining, scalloped line. It outlines the resorption front created during the previous phase of bone resorption. Following a resting phase, a seam of new bone (NB) is being deposited by osteoblasts (OB) within the periodontal ligament (PDL) that line the newly-formed bone. This new bone contains short, embedded Sharpey's fibers that help to reestablish the tooth anchorage to the bone. Following another resting phase, active bone resorption will resume.